Solve for $r$ : $r - 15 = -8$
Explanation: Add $15$ to both sides: $ \begin{eqnarray} \\ r - 15 &=& -8 \\ \\ {+15} && {+15} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-15 + r} &\hphantom{=}& \hphantom{-8} \\ r &=& -8 {+ 15} \\ \end{eqnarray} $ Simplify. $ r = 7$